Answers (3)

That "6" refers to a footnote, which is one form of citation. If you are reading something online, there is usually a link to the footnote. Some formats place footnotes at the bottom of the page where they cite, while other formats list them all at the end. Footnotes like other forms of citations are typically referenced at the end of the essay or book chapter or at the end of the book.

Haven't you checked, first of all, if the numbers are not hyperlinks? Because that could be the case if you didn't find the sources listed at the bottom of the page. This is of course far from classic forms of citation but happens frequently nowadays (if we're talking about an article which was only published online - can't check if that's the case because the link you provided doesn't lead to the article).